Friction stir welding (FSW) has various superior characteristics including a joining characteristic, and is applied to an aluminum alloy in various industrial fields soon after development. In the meantime, even regarding the friction stir welding of ferrous materials occupying many of structural materials, the study has been actively conducted recently. In the study stage, it is reported in Non-patent Literatures 1 to 3 that carbon steel is welded by friction stir welding.